Aquarium cart

The aquarium cart would be a water-filled glass container mounted on a cart. Similar to the boat or minecart, it would be able to hold and move mobs, but would work exclusively for aquatic ones.

Aquatic mobs like dolphins, nautiluses, squids, and turtles could be placed in the cart and moved around with leads attached to it, and due to the water inside, they wouldn't suffocate.

The cart could even be attached to a Happy Ghast like a boat, making aquatic mob transport less of a headache. Currently, moving entities like dolphins and nautiluses across long distances is especially painful, and I think this would solve the issue.